The invention relates to a composite material, in particular a composite membrane, in particular for gas separation, vapor separation, or pervaporation, comprising at least one polymer and a second polymer. The invention relates furthermore to a process for the production of a composite material, in particular a composite membrane.
Polymer membranes used commercially for gas separation are, as a rule, composite membranes which consist of a porous substructure and a pore-free, dense polymer film. In these, a composite material is used or the composite membrane comprises a corresponding composite material. For commercial utility it is essential to achieve high gas flows per unit area in order to keep the membrane surfaces and the energy costs low. The properties of the polymer predetermine the selectivity and the gas flow which are possible.
The flow decreases in proportion to increasing layer thickness while the selectivity remains essentially constant. It is therefore important to find suitable materials and process them into composite membranes, or integral asymmetric membranes, with the smallest possible layer thicknesses.
The available polymeric materials can be divided roughly into elastomers and vitreous polymers, which are distinguished by the position of their glass transition temperature above or below room temperature. Both types of polymer come into consideration for gas separation. Optimized membranes used commercially have, as a rule, separation-active layers with layer thicknesses of 0.5 to 1 μm.
It was possible to achieve an improvement of the separation properties of polymers known per se through the addition of porous materials. Through the addition of solids to the polymer a material arises with a mixed matrix of inorganic/organic components. Membranes produced according to this principle are known as “mixed matrix membranes” (see Koros, et al. U.S. Pat. No. 6,585,802, which is incorporated by reference herein). It was possible to improve the permeability as well as selectivity, in the ideal case even permeability and selectivity. Here the grain size of the porous structures used must be significantly less than the layer thickness desired in order to obtain dense films with embedded porous solids.
As porous solids to incorporate into the polymeric matrix, zeolites (molecular sieves), aluminum oxides, (boehmite, γ-aluminum oxide), silicon oxides, fullerenes (see Polotskaya et al., Fuller. Nanotub. Carbon Nanostruct. 12 (2004) 371-376, which is incorporated by reference herein) or carbon molecular sieves (see Vu et al., J. Membrane Sci. 211 (2003) 311-334, which is incorporated by reference herein) have already been investigated. Generally, the compatibility of the two components, polymer and porous filler, is very important. With incompatible materials (solid and polymer) flaws arise at the contact points, said flaws reducing the gas selectivity and thus making use in commercial application impossible. In general, elastomers can be processed to form flawless membranes more effectively than vitreous polymers.
Previously, zeolites (molecular sieves) above all were incorporated in elastomer membranes of polydimethylsiloxane (see D. Paul et al., J. Polymer Sci., 41 (1973) 79; M. Jia et al., J. Membrane Sci., 57 (1991) 289, which are incorporated by reference herein), where using zeolites Type 5A (Paul et al.) no improvement of the membrane properties was achieved. With zeolite Type ZSM-5 (Jia et al.) an increase of the permeability as well as of the selectivity was found. Zeolite ZSM-5 also yielded improved membranes on incorporation in cellulose acetate membranes (see Kulprathipanja et al. (U.S. Pat. Nos. 4,740,219 and 5,217,925), which is incorporated by reference herein). In other vitreous polymers, which are distinguished by particularly attractive gas selectivity, no clear increase of selectivity was found through a special treatment of the zeolites for the improvement of compatibility (see Gur, J. Membrane Sci. 93 (1994) 283; Duval et al., J. Membrane Sci. 80 (1993) 189, and J. Appl. Polymer Sci. 54 (1994) 409, which are incorporated by reference herein). Through the addition of softeners solid/polymer compatibility can be improved (see Mahajan, et al., J. Appl. Polym. Sci. 86 (2002) 881-890, which is incorporated by reference herein).
The addition of softeners does in fact improve the flawless incorporation of inorganic solids but the softener can, despite its low vapor pressure, bleed and volatize during the intensive rinsing with gas in the application of the membrane. Thereby the positive effect of the softener is gradually lost along with the improved gas separation properties. Thus it would not be possible to produce a membrane which is stable long-term.
Particularly important is the absolute freedom from flaws in the form of penetrating mesopores and macropores which allow a partial flow of the gas to occur without a separation effect. Even with a few flaws this partial flow reduces the selectivity considerably below the value which is required for implementation in commercial applications. The previously used, above-described classes of material have in common the fact that they are well compatible with only a few polymeric materials, for example, with the elastomer polydimethylsiloxane which however also has only a low selectivity with respect to O2/N2 or CO2/CH4. Vitreous polymers with usually good selectivities with respect to these gas pairs are not, as a rule, compatible with these inorganic, porous substances and also cannot necessarily be adapted to one another on the molecular level with auxiliary agents (see Mahajan, et al., Polym. Eng. Sci. 42 (2002) 1420-1431 and ibid. 1432-1441, which is incorporated by reference herein).